Benzene force fields

The present paper summarises the findings of our studies of force fields and vibrational frequencies of transition metal complexes. We discuss transition-metal-carbonyl complexes and complexes with small aromatic rings as ligands in detail. Benzene has an important role in this investigation as a Ugand, as well as an excellent benchmark test molecule. Accordingly, we also review the findings of our benzene force field in this report. 

Verification of the Method by the Reproduction of the Benchmark Benzene Force Field... 

Add the necessary control lines to obtain the full MM3 minimal input file and run the file under the MM3 force field to obtain the enthalpy of formation of the aromatic molecule benzene. 

Some authors refer to improper dihedrals when discussing dihedral torsion where we would not normally expect full rotation, for example, any of the C-C-C-C linkages in the benzene ring of aspirin. Many MM force fields treat... 

The final geometry optimizations with the DZ+D basis set and the analytic calculation of the force fields and MP-2 corrections were done with the program GRADSCfI on a CRAY-IA. There are 128 basis functions for the difluoro-benzenes, 132 basis functions for the dihydroxybenzenes and 136 basis functions for the diaminobenzenes. The calculations on the difluorobenzenes require about 14 x 10 non-zero 2e integrals whereas the calculations on the diaminobenzenes require about 40 x 10 non-zero 2e integrals. The integral sort step required for the... 

Based both on the determined isotopic shifts and the comparison of the radical IR spectrum with the spectra of various substituted benzenes, the bands have been assigned to the normal modes and the force field of the benzyl radical calculated (Table 8). 

Berces, A., Ziegler, T., 1992, The Harmonic Force Field of Benzene. A Local Density Functional Study , J. Chem. Phys., 98, 4793. 

While nonbonded atom pairs will typically not come within 1A of each other, it is possible for covalently bound pairs, either directly bounds, as in 1-2 pairs, or at the vertices of an angle, as in 1-3 pairs. Accordingly it may be considered desirable to omit the 1-2 and 1-3 dipole-dipole interactions as is commonly performed on additive force fields for the Coulombic and van der Waals terms. However, it has been shown that inclusion of the 1-2 and 1-3 dipole-dipole interactions is required to achieve anistropic molecular polarizabilites when using isotropic atomic polariz-abilites [50], For example, in a Drude model of benzene in which isotropic polarization was included on the carbons only inclusion of the 1-2 and 1-3 dipole-dipole interactions along with the appropriate damping of those interactions allowed for reproduction of the anisotropic molecular polarizability of the molecule [64], Thus, it may be considered desirable to include these short range interactions in a polarizable force field. 

Relaxation in Benzene V. CH(v=3) Dynamics Computed with a New Ab Initio Force Field. 

Stereo view of the three lowest energy minima of benzene obtained with the modified force field and the lowest energy minimum of benzamidine (thick lines for heavy atoms and thin lines for polar hydrogens) in the thrombin active site (thin lines). 

Bull et al. (97) reported a systematic 2H-NMR and MD study of siliceous faujasite. MD calculations were performed for 1 molecule of benzene adsorbed in a single unit cell of faujasite. Full framework flexibility was incorporated, using potential parameters from MSI s cff91 force field (5). Simulations were performed for diffusion at 298, 350,400, and 450 K, using a time step of 1 fs for a 25-ps calculation run (following 5 ps of equilibration). 

These deviations are the result of the fact that the MM2ERW force field makes explicit use of the rotational potential of 1,3-butadiene. Thus, cis- 1,3-butadiene is used as the appropriate reference conformation of butadiene for benzene and other aromatic molecules. The cis form of butadiene is about 3.5 kcal mol"1 higher in energy than the trans form,... 

German chemists synthesised the tetraspiroketone 1 and carried out theoretical calculations in an attempt to decide whether acid-catalysed isomerisation would lead preferentially or exclusively to the bispropellanone 2 or to the pentacyclic ketone 3. Force field calculations failed to reveal any significant preference for rearrangement, and hence experimental clarification was sought. The ketone 1 was unstable to acid, and was quantitatively isomerised to 3 within 30 minutes at 80°C when treated with one equivalent of a 0.25 M solution of anhydrous p-toluenesulfonic acid in benzene. 

Due to these large deviations the direct application of the SQMF method to the B3 molecule may lead to incorrect assignment of the vibrational modes. Therefore we performed a preliminary SQMF calculation on the benzene molecule, where the normal modes are well ascribed [1] and discriminated by symmetry. The extracted scale factors correspond to the set of symmetry-adapted internal coordinates, as introduced by Wilson [1], The same scale factors were then applied to the B3 molecule, which was considered as a system constructed by three benzene molecules. The single C-N bonds in B3 were treated with the same scale factors as the single C-H bonds in the benzene molecule. The scale factors corresponding to the N-H bonds were initially set equal to 1. The as-obtained scaled force-field, after minor adjustment of the scaling factors, was employed to calculate the normal frequencies of B3. Fig. 2(b) shows the corresponding pattern of the calculated scaled frequencies. It can be seen that there... 

Molecular-dynamic simulations are characterized by a solution of Newton s laws of motion for the molecules travelling through the zeolite pore system under control of the force field given by the properties of the host lattice, by interactions between the host and the molecules, and by interactions between the molecules. To date this has been possible only for the diffusion of simple molecules (e.g. methane or benzene) inside a zeolite lattice of limited dimensions [29, 37, 54], To take into account the effects of a chemical reaction as well would require quantum-mechanical considerations however, such simulations are in their infancy. 

Many applications of new force fields and new QM/MM methods of necessity focus on agreement with experimental or otherwise calculated results. Also in this section we will first show that DRF indeed gives a reliable model for static and response potentials and can lead to QM/MM—or even completely MM calculations—that are as good as, e.g., SCF calculations. To that end we point at some results for simple systems like the water and benzene dimers, and the three- and four-body interactions in several systems. 

---